I bought my car from a used car dealership and recieved to remotes for the keyless entry. I confirmed that all of the components are installed and that I merely need a remote....my car is a 98 323is, does anyone here have any experience buying one of these off ebay like the links below? Activation instructions?

I couldn't access the links you posted - but any E36 BMW remote with 2 oval buttons should work. Programming is easy, follow these instructions:

Preparation:
1. Close all doors, trunk, hood
2. Make sure the security system is disarmed and the key is out of the ignition.

Enter Code-Learning Mode:
3. Open the trunk and leave it open
4. Open the driver's door and sit in the driver's seat.
5. Close the driver's door.
6. Cycle the ignition switch 5 times between the off position and position 2 ignition ON, all dash warning lamps lit). The red status LED will illuminate continuously and the siren will chirp once to indicate that the code learning mode has been
initiated. (Do not start the engine. The ignition switch cycling in step 6 must be
performed within 10 seconds. The sequence in steps 1-6 must be performed
within 45 seconds.)

Register/Delete ID Codes:
7. Open driver's door (remain seated in driver's seat)
8. Close driver's door
9. Press and release any button on the remote you wish to register in the system. The status LED will shut off momentarily to indicate that one ID code has been registered.
10. Repeat steps 7-9 to register additional ID codes (up to a total of 4). If you register more than 4 remotes, each additional remote ID will
automatically delete the oldest remote ID code. If you lose a remote and want to make sure that only your replacement remote will work, register the new remote 4 times.

Exit Code-Learning Mode:
11. Open driver's door and exit from vehicle, leaving the door open
12. Close trunk
13. Close driver's door. The LED will turn off and the siren will chirp twice.
14. The procedure is now completed; test all remotes to confirm operation.